The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith, employs the oscillometric methodology. In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. Then, the cuff pressure is reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "oscillation complexes" or just simply "oscillations"). After suitable filtering to reject the DC component and to provide amplification, peak pulse amplitudes (PPA) above a given base-line are measured and stored. As the decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These amplitudes thus form an oscillometric blood pressure envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure ("MAP"). Systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use, the blood pressure cuff is placed on the patient and the operator sets a time interval, typically from 1 to 90 minutes, at which blood pressure measurements are to be made. The noninvasive blood pressure ("NIBP") monitor automatically starts a blood pressure determination at the end of the set time interval. Typically, the user selects a short interval if the patient is unstable since blood pressure may change to dangerous levels in a short time and selects a longer interval as the patient becomes more stable. The reason a short interval is not used in all cases is that the probability of trauma to the limb from the cuff inflation increases as the determination frequency increases.
Unfortunately, setting the interval for blood pressure measurement is not an exact science. If it is wrongly assumed that the patient's blood pressure is stable and a long interval is set, critical minutes may pass before dangerous changes in pressure are detected. Conversely, if it is wrongly assumed that the patient's blood pressure is volatile, the patient's limb is subjected to many unnecessary cuff inflations and the possible trauma resulting from such cuff inflations.
It is, accordingly, a primary object of the present invention to automatically determine when a blood pressure determination should be made without completely inflating the cuff and possibly causing trauma to the patient.
It is a further object of the present invention to automatically determine whether the patient's blood pressure has changed significantly since the last determination so that a new blood pressure determination may be instigated immediately.
It is also an object of the present invention to provide a technique for monitoring the status of the patient's blood pressure between determinations so that a change in status between blood pressure determinations will not go undetected.
If is a further object of the present invention to provide a technique for monitoring the status of the patient's blood pressure so that full blood pressure determinations need not be taken as frequently, thereby minimizing the possibility of trauma to the patient.